Prior to deploying xDSL transmissions, a subscriber loop is generally qualified or characterized by estimating the channel capacity, which depends on the transfer function of the network. The subscriber loop (or line) connects the customer premises (CP) to the central office (CO) and can be affected by a wide range of impairments, including but not limited to, load coils, bridge taps, mixed wire gauges, and bad splices. While the loop length and the wire gauge of the loop are generally not considered actual impairments, they also have a large impact on xDSL transmissions. Other impairments include split pairs, untwisted drop cables, radio-frequency interference (RFI), and cross-talk. Moreover, several of the aforementioned impairments for xDSL transmissions are not present for POTS (plain old telephony service) because xDSL exploits a much wider frequency band as compared to POTS. Consequently, the existing POTS testing equipment is not capable of qualifying a subscriber loop for xDSL transmission. Due to these impairments, the xDSL network termination (NT) installed at the CP may in some cases not even link up with the xDSL line termination (LT) in the DSL access multiplexer (DSLAM) at the CO. If the xDSL modems do link up with one another, there is no guarantee with respect to the quality-of-service (QoS) in terms of bit rate.
Qualifying a subscriber loop for xDSL requires estimating its channel capacity, which depends on the attenuation of the line and also on the noise power spectral density (PSD) at the CO for upstream transmission and at the CP for downstream transmission, respectively. The estimate of the channel capacity of a particular loop/line will be most accurate if the transfer function of the line and the noise PSD at the CO and CP are measured directly.
Conventional methods for qualifying a subscriber loop include use of xDSL test units available on the market that are capable of performing such measurements. In addition, these test units are often combined with a “golden” modem plug-in module that emulates a real xDSL modem of a certain type, such as ADSL, in order to estimate the real bit rate instead of only the theoretical channel capacity. However, this approach requires sending a technician to the CP, which is very expensive. Single-ended loop testing (SELT) can be used to extract information about the transmission environment (e.g., the loop) in a DSL system by performing reflective measurements remotely at the CO/CP (or Modem) terminal, without the need to dispatch a technician. As an example, SELT may comprise injecting signals into a loop under test at a central office (CO) in order to determine the loop capability for supporting different kinds of DSL services. As such, SELT often plays an important role in DSL provisioning and maintenance.